Prothrombin, a key enzyme in the blood clotting process, contains 10 glutamic acid residues that are gamma-carboxylated (by vitamin K dependent reaction) near the N-terminus end. This modification of the glu residues is mandatory for binding Ca ions and the phospholipid, and to its ultimate cleavage to form the active thrombin. We have isolated and purified the fragment 1 which is the polypeptide chain of 156 residues at the N-terminal end of prothrombin. The piece contains all the gamma-carboxy glutamic acid residues and binds 10-12 moles of calcium. We propose to determine the three-dimensional structure of this fragment to characterize the Ca binding sites using single crystal x-ray diffraction methods.